It is well known to provide electrical and electronic devices, such as electric motors and computers, with a variable voltage power supply by varying the duty cycle of a power converter attached to an initial voltage source. For example, a five volt direct current (i.e., DC) source may be effectively converted into a 2 volt output by simply using a switch, such as a transistor, to turn the current on for 40 milliseconds (i.e., 40 ms) and then off for 60 ms, etc. This would be known as a 40% duty cycle. The 5 volt square wave output of the transistor, typically either a Bipolar transistor or a MOS transistor, after passing through a capacitor, resistor and inductor network to smooth out variations, will effectively result in an DC voltage of 0.40 times 5 volts, or 2 volts DC. The same method of varying the duty cycle may be used to provide essentially any desired voltage level up to the value of the initial voltage, in this example 5 volts DC. This type of apparatus is known as a power converter.
Alternatively, the initial power supply may be an alternating current (i.e., AC) and not a DC supply. AC voltages are typically 120 to 240 volts, and so it is common to use a transformer to step down the voltage to a level more compatible with electronic devices. In this example, the 120 volt primary side of a transformer might have a coil with 24 turns, and the secondary side transformer coil would then have only a single turn, and thus provide 5 volts to a switch on the secondary side of the transformer. Then, as in the previous example, the duty cycle is selected to provide the desired output voltage. With a cycle rate Q used to synchronize both sides of the transformer resulting in a peak to peak time period of Tau, and a switch on period of xe2x80x9ctxe2x80x9d, the output voltage of the power converter is then given by the expression Vout=Vin X (t/Tau). Note that the AC supply is generally transformed into a pseudo DC voltage by the use of diodes on the secondary side of the transformer, which convert the sinusoidal waves into positive voltage square waves, plus the use of smoothing filters.
In order to match the maximum amount of power needed in any particular circuit or electrical device to the maximum power limit of the individual power converters, it is common to use multiple power converters connected in parallel to achieve the desired current capacity. This arrangement is useful because there is an increased current flow on the secondary side in a voltage step down transformer, and thus a possibility of power converter over heating and burnout exists. A problem that exists with this parallel arrangement of power supplies is that the parallel power supplies may not share the load precisely. When the output load is small, then it is possible that one of the parallel power converters may not be exactly as strong as the others, and power may feedback through that one power converter to the transformer. This is a positive feedback mechanism and may result in the higher powered converters drawing more and more power until burnout occurs. This problem is known as current hogging, and it would be a benefit to provide an arrangement of parallel power converters that did not suffer from current hogging when operating in a state where the output current demands are low.
An apparatus for providing synchronous power to an electrical or electronic device without current hogging, comprises power converters connected in parallel, with each converter having a circuit for sensing the current flow direction and value at either a location on the primary side of the transformer, or on the secondary side of the transformer. In an embodiment of the invention, each one of the sensing circuits can switch the converter to an inactive state when current hogging causes feedback that may reach the transformer and cause a positive feedback cycle. With such an arrangement the feedback loop and consequent overheating and burnout may be avoided by disabling the affected converter.
In a preferred embodiment of the invention, the converters are all synchronized to the primary transformer side modulator using a first series connected MOSFET, and a second parallel connected MOSFET catch transistor. The catch FET is preferably either 180 degrees out of phase with the modulator, or driven by a pulse width modulator. The feedback cutoff is preferably implemented by an additional driver for the second transistor which is controlled by the output of an operational amplifier comparing the current sensor value versus a reference voltage.